KINETICS OF ELECTRON-TRANSFER REACTIONS INVOLVING THE RU(CN)64-/3- COUPLE IN AQUEOUS-MEDIA

The kinetics and mechanisms of electron-transfer reactions involving the Ru(CC)64−and Ru(CN)63−ions in aqueous media have been investigated. The electron-self-exchange rate constant has been directly measured for the Ru(CN)64−/3−couple from13C NMR line-broadening experiments using13C-enriched complexes. At 25 ° C and an ionic strength of 0.10 M (NaC1O4), k11= (8.3 ± 0.4) × 103M−1s−1with ΔH* = 40 ± 5 kj mol−1and ΔS* = −36 ± 15 J K−1mol−1. The kinetics of the oxidation of the Ru(CN)64−ion by IrCl62−and CoW12O405−and the reduction of the Ru(CN)63−ion by NO2−, I−, Ni([9]aneN3)22+, and IrCl63−have been studied by using stopped-flow techniques. The cross-reaction data have been treated according to the Marcus relations and yield a self-exchange constant of 104.0±0.8M−1s−1(25.0 ° C, I = 0.10 M) for the Ru(CN)64−/3−couple. The directly and indirectly determined electron-exchange rate constants agree well with the value of (2 ± 1) × 104M−1s−, predicted by semiclassical model calculations. The rate constants are compared with the analogous values for a number of small, low-spin d6/d5iron and ruthenium complex couples and are discussed in terms of the relative contributions of the inner- and solvent-sphere reorganization barriers to electron exchange. © 1990, American Chemical Society. All rights reserved.